1. Field of the Invention
The present invention relates to computer networks and more particularly to dynamically responding to event-based traffic redirection in a computer network
2. Background Information
A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations. Many types of networks are available, with the types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol consists of a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network.
Since management of interconnected computer networks can prove burdensome, smaller groups of computer networks may be maintained as routing domains or autonomous systems. The networks within an autonomous system (AS) are typically coupled together by conventional “intradomain” routers configured to execute intradomain routing protocols, and are generally subject to a common authority. To improve routing scalability, a service provider (e.g., an ISP) may divide an AS into multiple “areas.” It may be desirable, however, to increase the number of nodes capable of exchanging data; in this case, interdomain routers executing interdomain routing protocols are used to interconnect nodes of the various ASes. Moreover, it may be desirable to interconnect various ASes that operate under different administrative domains. As used herein, an AS or an area is generally referred to as a “domain,” and a router that interconnects different domains together is generally referred to as a “border router.”
An example of an interdomain routing protocol is the Border Gateway Protocol version 4 (BGP), which performs routing between domains (ASes) by exchanging routing and reachability information among neighboring interdomain routers of the systems. An adjacency is a relationship formed between selected neighboring (peer) routers for the purpose of exchanging routing information messages and abstracting the network topology. The routing information exchanged by BGP peer routers typically includes destination address prefixes, i.e., the portions of destination addresses used by the routing protocol to render routing (“next hop”) decisions. Examples of such destination addresses include IP version 4 (IPv4) and version 6 (IPv6) addresses. BGP generally operates over a reliable transport protocol, such as TCP, to establish a TCP connection/session. The BGP protocol is well known and generally described in Request for Comments (RFC) 1771, entitled A Border Gateway Protocol 4 (BGP-4), published March 1995.
Examples of an intradomain routing protocol, or an interior gateway protocol (IGP), are the Open Shortest Path First (OSPF) routing protocol and the Intermediate-System-to-Intermediate-System (IS-IS) routing protocol. The OSPF and IS-IS protocols are based on link-state technology and, therefore, are commonly referred to as link-state routing protocols. Link-state protocols define the manner with which routing information and network-topology information are exchanged and processed in a domain. This information is generally directed to an intradomain router's local state (e.g., the router's usable interfaces and reachable neighbors or adjacencies). The OSPF protocol is described in RFC 2328, entitled OSPF Version 2, dated April 1998 and the IS-IS protocol used in the context of IP is described in RFC 1195, entitled Use of OSI IS-IS for routing in TCP/IP and Dual Environments, dated December 1990, both of which are hereby incorporated by reference.
An intermediate network node often stores its routing information in a routing table maintained and managed by a routing information base (RIB). The routing table is a searchable data structure in which network addresses are mapped to their associated routing information. However, those skilled in the art will understand that the routing table need not be organized as a table, and alternatively may be another type of searchable data structure. Although the intermediate network node's routing table may be configured with a predetermined set of routing information, the node also may dynamically acquire (“learn”) network routing information as it sends and receives data packets. When a packet is received at the intermediate network node, the packet's destination address may be used to identify a routing table entry containing routing information associated with the received packet. Among other things, the packet's routing information indicates the packet's next-hop address.
To ensure that its routing table contains up-to-date routing information, the intermediate network node may cooperate with other intermediate nodes to disseminate routing information representative of the current network topology. For example, suppose the intermediate network node detects that one of its neighboring nodes (i.e., adjacent network nodes) becomes unavailable, e.g., due to a link failure or the neighboring node going “off-line,” etc. In this situation, the intermediate network node can update the routing information stored in its routing table to ensure that data packets are not routed to the unavailable network node. Furthermore, the intermediate node also may communicate this change in network topology to the other intermediate network nodes so they, too, can update their local routing tables and bypass the unavailable node. In this manner, each of the intermediate network nodes becomes “aware” of the change in topology.
Typically, routing information is disseminated among the intermediate network nodes in accordance with a predetermined network communication protocol, such as a link-state protocol (e.g., IS-IS, or OSPF). Conventional link-state protocols use link-state advertisements or link-state packets (or “IGP Advertisements”) for exchanging routing information between interconnected intermediate network nodes (IGP nodes). As used herein, an IGP Advertisement generally describes any message used by an IGP routing protocol for communicating routing information among interconnected IGP nodes, i.e., routers and switches. Operationally, a first IGP node may generate an IGP Advertisement and “flood” (i.e., transmit) the packet over each of its network interfaces coupled to other IGP nodes. Thereafter, a second IGP node may receive the flooded IGP Advertisement and update its routing table based on routing information contained in the received IGP Advertisement. Next, the second IGP node may flood the received IGP Advertisement over each of its network interfaces, except for the interface at which the IGP Advertisement was received. This flooding process may be repeated until each interconnected IGP node has received the IGP Advertisement and updated its local routing table.
Multi-Protocol Label Switching (MPLS) Traffic Engineering has been developed to meet data networking requirements such as guaranteed available bandwidth or fast restoration. MPLS Traffic Engineering exploits modern label switching techniques to build guaranteed bandwidth end-to-end tunnels through an IP/MPLS network of label switched routers (LSRs). These tunnels are a type of label switched path (LSP) and thus are generally referred to as MPLS Traffic Engineering (TE) LSPs. Examples of MPLS TE can be found in RFC 3209, entitled RSVP-TE: Extensions to RSVP for LSP Tunnels dated December 2001, RFC 3784 entitled Intermediate-System-to-Intermediate-System (IS-IS) Extensions for Traffic Engineering (TE) dated June 2004, and RFC 3630, entitled Traffic Engineering (TE) Extensions to OSPF Version 2 dated September 2003, the contents of all of which are hereby incorporated by reference in their entirety.
Establishment of an MPLS TE-LSP from a head-end LSR to a tail-end LSR involves computation of a path through a network of LSRs. Optimally, the computed path is the “shortest” path, as measured in some metric, that satisfies all relevant LSP Traffic Engineering constraints such as e.g., required bandwidth, “affinities” (administrative constraints to avoid or include certain links), etc. Path computation can either be performed by the head-end LSR or by some other entity operating as a path computation element (PCE) not co-located on the head-end LSR. The head-end LSR (or a PCE) exploits its knowledge of network topology and resources available on each link to perform the path computation according to the LSP Traffic Engineering constraints. Various path computation methodologies are available including CSPF (constrained shortest path first). MPLS TE-LSPs can be configured within a single domain, e.g., area, level, or AS, or may also span multiple domains, e.g., areas, levels, or ASes.
The PCE is an entity having the capability to compute paths between any nodes of which the PCE is aware in an AS or area. PCEs are especially useful in that they are more cognizant of network traffic and path selection within their AS or area, and thus may be used for more optimal path computation. A head-end LSR may further operate as a path computation client (PCC) configured to send a path computation request to the PCE, and receive a response with the computed path, potentially taking into consideration other path computation requests from other PCCs. It is important to note that when one PCE sends a request to another PCE, it acts as a PCC.
Some applications may incorporate unidirectional data flows configured to transfer time-sensitive traffic from a source (sender) in a computer network to a destination (receiver) in the network in accordance with a certain “quality of service” (QoS). Here, network resources may be reserved for the unidirectional flow to ensure that the QoS associated with the data flow is maintained. The Resource ReSerVation Protocol (RSVP) is a network-control protocol that enables applications to reserve resources in order to obtain special QoS for their data flows. RSVP works in conjunction with routing protocols to, e.g., reserve resources for a data flow in a computer network in order to establish a level of QoS required by the data flow. RSVP is defined in R. Braden, et al., Resource ReSerVation Protocol (RSVP), RFC 2205. In the case of traffic engineering applications, RSVP signaling is used to establish a TE-LSP and to convey various TE-LSP attributes to routers, such as border routers, along the TE-LSP obeying the set of required constraints whose path may have been computed by various means.
Generally, TE-LSPs are configured with static (i.e., fixed) constraints, such as bandwidth size. A system administrator with knowledge of the network characteristics may configure the TE-LSPs in accordance with desired capabilities, such as, e.g., to handle a known maximum average load level. For instance, if traffic traversing a particular TE-LSP has a daily maximum value of 5 Megabytes/second (MB), the administrator may configure the TE-LSP to reserve 5 MB.
One problem with statically configured TE-LSPs is that they do not adapt to changes in traffic patterns over the TE-LSP. In some cases, the traffic traversing a TE-LSP may significantly vary upon the time of day, days in a week, etc. Various algorithms have been proposed to adequately and dynamically resize a TE-LSP based on the measured traffic load (e.g., with low pass filters). For example, a resizing algorithm may consist of measuring or “sampling” the average load (bandwidth) for a short period of time (e.g., a sample rate or frequency of 5 mins). The TE-LSP may be resized after a longer period of time (e.g., a resize frequency of 1 hour) based on the maximum average load of the sampled periods for the past resizing period.
A common practice in TE-enabled networks consists of deploying a mesh of TE-LSPs between a plurality of edge devices (provider edge, or PE routers) through a core network of fewer (generally large capacity) routers (provider, or P routers). In a mesh (e.g., a “full mesh”), each PE router on one side of the core is connected to each PE router on the other side of the core via a TE-LSP. The mesh of TE-LSPs provides various benefits within the network, as known to those skilled in the art. In certain network configurations (e.g., with a large number of PE routers), however, this results in a large number of TE-LSPs throughout the network. For example, in the event there are 100 PE routers on each side of the core network, a total of 1000 TE-LSPs are necessary to create a full mesh. Generally, there are more (e.g., 5 to 10 times more) PE routers than there are P routers in the network, so one solution to limit the number of TE-LSPs in the network has been to create a mesh of TE-LSPs between the P routers, and not the PE routers. This may significantly reduce the number of TE-LSPs, such as by a factor of, e.g., 25-100. The PE routers may then communicate with the P routers through conventional routing, e.g., IP or MPLS routing.
Occasionally, a network element (e.g., a node or link) will fail, causing redirection of the traffic that originally traversed the failed network element to other network elements that bypass the failure. Generally, notice of this failure is relayed to the nodes in the same domain through an advertisement of the new network topology, e.g., an IGP Advertisement, and routing tables are updated to avoid the failure accordingly. Typically, both IP traffic and any TE-LSPs are redirected to avoid a failure in a manner known to those skilled in the art. In the above case where a mesh of TE-LSPs are provided between P routers, if any one of the P routers fails, all TE-LSPs originating at that P router also fails. As the network becomes aware of the change in topology and converges, the PE routers start redirecting their traffic to another available P router in the core in order to reach the other side. This condition leads to a burst of traffic redirected to TE-LSPs originating at other P routers (also referred to in the art as “traffic sloshing”). These TE-LSPs, however, may not be appropriately sized to handle the large burst of new traffic, thus rendering the traffic engineering mechanisms inaccurate and less efficient during the failure of a P router.
Particularly, a statically configured TE-LSP is unable to adapt to any changes in traffic patterns, much less a burst of traffic due to a failure of a P router. One solution to this static TE-LSP limitation is to configure each TE-LSP with a size that could handle such a failure; however to do so the TE-LSP would need to be much larger than necessary for conventional traffic, and hence would be an inefficient use of network resources. A dynamically sized TE-LSP, on the other hand, would eventually resize to an appropriate size, depending upon the sample/resize frequencies of the TE-LSP. Slow sample/resize frequencies would not react efficiently (e.g., quickly) enough to compensate for the burst of traffic. Conversely, fast sample/resize frequencies would react efficiently to the burst of traffic, yet, during steady states, the faster frequencies may create more frequent changes to the TE-LSP, resulting in excess signaling messages and possible network instability. Also, when a failed node is restored or a new node is added, the TE-LSPs that were resized to compensate for the redirected traffic may have reserved the available resources of the network in a way that prevents the restored/new node from establishing the appropriate TE-LSPs, leading to double booking problems (at least temporarily).